13-06-2013, 02:26 PM
Alloy Steels
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Introduction :
Steels are, essentially, alloys of iron and carbon, containing up to 1.5 %
of carbon. Steel is made by oxidizing away the impurities that are present in
the iron produced in the blast furnace.
The earliest attempt to produce an alloy steel was in 1822 and it has
progressing in producing the alloy steel because of using alloy steel in those
industries upon modern civilization largely depends.
Pure metal objects are used where good electrical conductivity, good
thermal conductivity, good corrosion resistance or a combination of these
properties are required. Therefore alloys are mainly used for structural
materials since they can be formulated to give superior mechanical
properties.
It is called as alloy steel because there are other elements added to the
iron beside the carbon with specific amount for each element. These
elements improve the properties of the alloying steel and make it used with
applications more than the carbon steel.
Engineering Materials
Alloys steels are generally classified into two major types depending on
the structural classification :
- Low alloy steels :
It is one that possesses similar microstructure to, and requires similar heat
treatment to, plain carbon steels. These generally contain up to 3 4 % of
one or more alloying elements for purpose of increasing strength, toughness
and hardenability. The applications of low alloy steels are similar to those of
plain carbon steels of similar carbon contents. Low alloy steels containing
nickel are particularly suitable for applications requiring resistance to
fatigue.
- High alloy steels :
Those steels that possess structures, and require heat treatments, that
differ considerably form those of plain carbon steels. A few examples of
high alloy steels are given below:
High-speed tool steels
Tungsten and chromium form very hard and stable carbides. Both
elements also raise the critical temperatures and, also, cause an increase in
softening temperatures. High carbon steels rich in these elements provide
hard wearing metal-cutting tools, which retain their high hardness at
temperature up to 600˚C. a widely used high-speed tool steel composition is
containing 18% of tungsten, 4% of chromium, 1% of vanadium and 0.8% of
carbon .
This high-alloy content martensite dose not soften appreciably unit it is
heated at temperatures is excess of 600˚C making them usable as cutting
tools at high cutting speeds.
Stainless steels
When chromium is present in amounts in excess of 12% , the steel
becomes highly resistance to corrosion, owing to protective film of
chromium oxide that forms on the metal surface. Chromium also raises the á
to ã transformation temperature of iron, and tends to stabilize ferrite in the
structure.
Maraging steels
These are very high-strength materials that can be hardened to give
tensile strengths up to 1900 MN/m2. They contain 18% of nickel, 7% of
cobalt and small amounts of other elements such as titanium, and the carbon
content is low, generally less than 0.05% .
A major advantage of marging steels is that after the solution treatment they
are soft enough to be worked and machined with comparative ease.
The effect on grain growth.
The rate of crystal growth is accelerated, particularly at high
temperatures, by the presence of some elements, notably chromium.
Fortunately, grain growth is retarded by other elements, notably
nickel and vanadium, whose presence-thus produce a steel which is less
sensitive to the temperature conditions of heat-treatment.
The Retardation of Transformation Rates.
By adding alloying elements, we reduce the critical cooling rate which is
necessary for the transformation of austenite to martensite to take place. This
feature of the alloying of steels has obvious advantages and all alloying
elements, with the exception of cobalt, will reduce transformation rates.
In order to obtain a completely martensitic structure in the case of a plain
0.83% carbon steel, we must cool it from above 723˚ C to room temperature
in approximately one second. This treatment involves a very drastic quench,
generally leading to distortion or cracking of the component. By adding small
amounts of suitable alloying elements, such as nickel and chromium, we
reduce this critical cooling rate to such an extent that a less drastic oilquench
is rapid enough to produce a totally martensitic structure. Further
increases in the amounts of alloying elements will so reduce the rate of
transformation that such a steel can be hardened by cooling in air.